DT770s will have more bass emphasis and slightly recessed mids in comparison to the K550s. Both have good subbass extension (i.e. play deep when needed). If you consider yourself more of a basshead, then the DT770s might be the better choice. If not, then perhaps the K550s.
K551s are essentially the same as the K550s but with remote control and mic (you might consider the K550s instead if they are cheaper). I think the K550s are very durable. And see this recent quote from the AKG K550 discussion thread (great place to learn more):
Originally Posted by sludgeogre
Again, very glad I could help, it has taken me a long time to decide on what gear I like and want. You brought up a good point, though, which is the design of the headphone. They are beautiful, extremely comfortable, and built like tanks. I throw these into my backpack every day and they still look brand new after a couple months. They are really comfortable for long listening sessions and they don't make you look totally ridiculous.
I think I'll go for the K550s over the DT770s and K551s, because they supposedly have a better wire and are more durable. Also I don't need the remote control and mic. Edited by problematiic - 5/1/14 at 8:24am